Rainwater Collection Calculator
Calculate how much rainwater you can harvest from your roof
Inputs
Footprint of roof draining to collection
Average yearly rainfall in your area
Metal roofs ~90%, shingles ~75%
Collection per inch of rain
Roof efficiency
US rainfall averages
How to use this calculator
- 01Measure or estimate your roof collection area (footprint, not surface)
- 02Look up average annual rainfall for your location
- 03Estimate collection efficiency based on your roof type
- 04View your potential annual rainwater harvest
Rainwater harvesting tips
A 1,000 sq ft roof can collect about 600 gallons from just 1 inch of rain. Metal roofs have the highest collection efficiency (90%+).
First flush diverters help improve water quality by discarding the first few gallons that wash debris off the roof.
Storage sizing: Size your tank based on the longest dry spell in your area. In many regions, 2-4 weeks of storage covers dry periods between rain events.
Frequently asked questions
How do I calculate roof collection area?+
Use the footprint (horizontal area), not the actual roof surface area. A 30x40 foot house has 1,200 sq ft of collection area regardless of roof pitch. Include all areas that drain to collection.
What affects collection efficiency?+
Metal roofs are most efficient (90%+). Asphalt shingles lose more to absorption (75-80%). First flush diverters, gutter losses, and evaporation also reduce total collection.
Is rainwater safe to drink?+
Rainwater should be filtered and treated before drinking. A basic system includes first flush diverter, sediment filter, and UV treatment or chlorination. Always test water quality.
Result
19,936gallons
Total yearly potential